Phosphoric acid is a very versatile product finding its way into diverse industries. By far the largest market for phosphoric acid is in the fertilizer industry where this acid is neutralized with ammonia to provide a rich source of nitrogen and phosphorous readily assimilable by food crops. Nitrogen and phosphorous have important nutritional values, the most notable perhaps being as structural components of nucleic acid. Accordingly, providing an adequate supply of nitrogen to food crops facilitates cell multiplication, and thus plant growth.
It is important that phosphoric acid used in fertilizer production not be contaminated with excessive concentrations of metal-containing compounds, notably iron and aluminum compounds. Excessive concentrations of such compounds can make it impossible to meet commercial grade specifications for the ammonium phosphate produced from the phosphoric acid. Also, iron and aluminum compounds tend to precipitate slowly from phosphoric acid in the form of unfilterable sludges, which have a tendency to settle in storage tanks and rail cars and thereby interfere with the storage and transportation of the acid, and with its ultimate use in fertilizer production. The presence of high levels of aluminum is particularly deleterious; unlike iron, aluminum has no nutrient value, and in fact can be toxic to plants and inhibit root growth in acid soils.
The iron and aluminum content of wet process acid is derived from the phosphate rock from which said acid is produced. Phosphoric acid produced using currently available technology from phosphate rocks with unusually high iron and aluminum contents, such as the nonsedimentary rocks found in Sri Lanka, has a (Fe.sub.2 O.sub.3 +Al.sub.2 O.sub.3)/P.sub.2 O.sub.5 weight ratio in excess of 0.085 and is therefore not suitable for fertilizer production.
Most phosphoric acid is currently produced from sedimentary phosphate rock mined in Florida and in Morocco; the predominant technology used in phosphoric acid production is a wet process in which calcium sulfate dihydrate (gypsum) is generated as a by-product. This process is usually referred to as the dihydrate process. In this process ground phosphate ore, commonly referred to in the phosphoric acid industry as phosphate rock, is reacted with sulfuric acid in the presence of recycled dilute phosphoric acid. The slurry resulting from the chemical reaction between phosphate rock and sulfuric acid comprises orthophosphoric acid (commonly known in the industry as phosphoric acid or as wet process phosphoric acid), gypsum, and numerous suspended and dissolved impurities. The slurry is filtered to remove most of the gypsum and other suspended impurities. The resulting filtrate usually contains between 25 and 32% by weight P.sub.2 O.sub.5, between about 1 and 8% by weight gypsum and other suspended impurities which are not removed by filtering, and dissolved impurities, including the aforementioned iron and aluminum compounds. This dilute wet process phosphoric acid is usually concentrated in multiple stage evaporators to a P.sub.2 O.sub.5 content between about 45% to 55% prior to further storage and eventual shipment and/or conversion to ammoniated products.
The problems encountered in the industry as a result of phosphate rock - derived impurities reporting to the phosphoric acid are well known. First and foremost, the metal impurity content of the wet process acid must be sufficiently low, so that upon its ammoniation the resulting diammonium phosphate (DAP) meets the commercial grade specification, i.e. has a minimum nitrogen content of 18% and a minimum P.sub.2 O.sub.5 content of 46%. Such a product is commonly referred to in the fertilizer industry as 18-46-0 DAP. Another serious problem is that of sludge formation which is attributed primarily to a phenomenon known in the industry as post- precipitation. The sludge is difficult and costly to remove from the phosphoric acid and, additionally, represents substantial losses of P.sub.2 O.sub.5 values. One of the major constituents of the sludge is a complex aluminum-iron-phosphate salt, (Al,Fe).sub.3 KH.sub.14 (PO.sub.4).sub.8.4H.sub.2 O. The sludge not only impairs the storage and transportation of wet process phosphoric acid, but its high phosphate content can result in substantial P.sub.2 O.sub.5 losses. The post-precipitation phenomenon is particularly acute at the higher P.sub.2 O.sub.5 concentrations attained through multiple stage evaporation of dilute wet process acid.
In the case of phosphoric acid produced from phosphate rocks having high iron and aluminum contents and low levels of magnesium, the metal impurity content of the acid may be conveniently expressed in terms of its (Fe.sub.2 O.sub.3 +Al.sub.2 O.sub.3)/P.sub.2 O.sub.5 weight ratio. Phosphoric acid derived from nonsedimentary phosphate rocks such as those found in Sri Lanka should have a (Fe.sub.2 O.sub.3 +Al.sub.2 O.sub.3)/P.sub.2 O.sub.5 weight ratio equal to or less than 0.085 in order for the acid to be suitable for the production of DAP which meets commercial specifications for nitrogen and P.sub.2 O.sub.5 content.
Because of the commercial importance of wet process phosphoric acid and the adverse economic impact associated with the presence of excessive concentrations of metallic impurities in this product, a considerable technological effort has been made to develop methods for purifying wet process phosphoric acid and for reducing post-precipitation in such acid.
Settling is often employed to reduce the sludge content of wet process phosphoric acid prior to shipment. Settling of sludge solids, however, does not resolve the post-precipitation problem easily or economically because of the lengthy time required, the loss of P.sub.2 O.sub.5 values associated with the sludge, and because the clarified acid may continue to exhibit a tendency for post- precipitation. Purification schemes, such as solvent extraction and ion exchange processes, have not found commercial acceptance because of excessive capital and operating costs.
Another strategy proposed in the technical literature entails chemical treatment of wet process phosphoric acid to stabilize it against post-precipitation. Examples of stabilization strategies can be found in U.S. patents issued to Richard Hill, including U.S. Pat. Nos. 4,110,422; 4,164,550; 4,248,846; 4,279,877; and 4,293,311.
U.S. Pat. Nos. 4,110,422 and 4,164,550 describe a process in which stabilized wet process phosphoric acid is produced by addition of an aluminum silicate material, such as perlite, to clarified dilute phosphoric acid, concentrating the acid, transferring it to a crystallization zone where additional clarification occurs, and then further concentrating the acid. This process is not directed toward reducing the aluminum content of the acid and may leave an excessive concentration of aluminum in the acid product.
U.S. Pat. No. 4,248,846 further incorporates a recycle stream from the crystallizer underflow to the acid train and provides for the addition of sulfuric acid to an evaporator when processing rock high in both iron and aluminum. This process also produces an acid which may be high in aluminum.
U.S. Pat. No. 4,279,877 provides a process for high-iron feed acid in which some of the iron is present in the ferrous form. The patent teaches the use of an oxidant, such as hydrogen peroxide, to oxidize all ferrous iron to the ferric state. The treatment reduces post-precipitation of the final product acid, but the final product may still be high in aluminum.
U.S. Pat. No. 4,293,311 modifies the process of U.S. Pat. No. 4,110,422 by dividing the crystallizer underflow into two streams and recycling said streams to specific points in the process. Aluminum silicate is still required and this process produces acids which may still have a high aluminum content.
More selective chemical purification schemes are also known. U.S. Pat. No. 2,954,287 to Carothers et al teaches the production of wet process acid by the addition of an alkali salt to the sulfuric acid used to attack high grade Florida phosphate rock. Impurities are advantageously precipitated, it is taught. However, the product acid may contain undesirably high concentrations of alkali metal.
U.S. Pat. No. 4,435,372 to Frazier et al describes a complex method of removing aluminum, magnesium, and fluoride impurities from wet process phosphoric acid with the calcium sulfate hemihydrate filter cake by hydrolyzing and recycling the off-gas scrubber solutions in the presence of a ferric ion catalyst. This catalytic process appears complicated and is applicable only to phosphoric acid having a P.sub.2 O.sub.5 content greater than about 40%.
U.S. Pat. No. 4,136,199 to Mills describes a method of removing metal ion impurities, such as aluminum, from phosphoric acid by adding an impure sludge which contains calcium fluoride and which is obtained by treating waste pond water with lime or limestone. The process is applicable to concentrated wet process phosphoric acid having a P.sub.2 O.sub.5 content of 38% to 54%. In one embodiment, the acid is mixed with a calcium fluoride-containing sludge and the resulting mixture is aged for five days, following which it is ultracentrifuged. Relatively high levels of aluminum remain in the phosphoric acid and the overall process is difficult to control because of varying compositions of the sludges used.
U.S. Pat. No. 4,379,776 to Beer et al teaches the removal of aluminum from phosphoric acid by precipitating aluminum fluorophosphate from the acid. The principal disadvantage of this approach is the loss of phosphate values with the aluminum-containing precipitate.
U.S. Pat. No. 4,299,804 to Parks et al teaches a process for precipitating magnesium and aluminum impurities from unconcentrated acids, since the high viscosity of concentrated wet process acid makes phase separation difficult. The patent teaches the addition to the filter grade wet process acid of a fluoride ion donating compound which may be hydrofluoric acid, sodium fluoride, sodium bifluoride or ammonium fluoride. Silica in the acid is taught as inhibiting or preventing the precipitation of metal impurities out of solution.
U.S. Pat. No. 4,710,366 to Astley et al teaches a method for producing wet process phosphoric acid with low post-precipitation characteristics and with reduced aluminum and magnesium levels from a dilute wet process acid by mixing fluosilicic acid with said dilute wet process acid, concentrating the mixture to about 50% P.sub.2 O.sub.5, subjecting it to crystallization, clarification, and further evaporation to a P.sub.2 O.sub.5 content of about 60%.
None of the methods of prior art extend the utility of the conventional dihydrate wet process phosphoric acid production technology to the processing of phosphate rocks having unusually high iron and aluminum contents, such as the nonsedimentary rocks occuring in Sri Lanka. The reason for this limitation in the applicability of the prior art methods cited hereinabove lies in the fact that said methods were generally developed to control or minimize post-precipitation in wet process phosphoric acid without necessarily reducing its iron and aluminum content. Thus, while stabilizing an acid against post-precipitation, such methods do not necessarily render an acid suitable for conversion to diammonium phosphate which meets commercial grade specifications. Another disadvantage characterizing most of the prior art methods stems from the fact that these methods are generally applied to the phosphoric acid after it is produced, not while it is produced. Thus, such methods suffer from the need for additional processing steps and corresponding additional equipment and operating costs.
It is therefore an object of this invention to provide a method for removing aluminum contamination from phosphoric acid during the dihydrate process.
It is another object of this invention to provide a method for wet process phosphoric acid production from nonsedimentary phosphate rock having an unusually high iron and aluminum content, such as the igneous or metamorphic rock found in Sri Lanka, wherein said wet process phosphoric acid is suitable for conversion to diammonium phosphate which meets commercial specifications for nitrogen and P.sub.2 O.sub.5 content.
It is still another object of this invention to provide an improvement to the dihydrate process in order to widen the scope of its applicability to nonsedimentary types of phosphate rocks.